The present invention relates to a precast refractory shape for use in the handling of molten metal, and particularly to a precast slag/iron interface in a blast furnace trough.
In the production of metal articles, molten metal is often transferred from a first vessel through a trough. The trough will have at least one outlet through which the molten metal can pass to a second vessel or mold. For example, the production of iron comprises transferring molten iron from a blast furnace through a trough to a ladle.
A trough includes a metal shell having a refractory lining appropriate to the type of metal being cast. For iron, the lining often begins with a bed of a dry vibratable material over which precast blocks are set. Temporary forms are placed in the trough to define its interior shape. A castable refractory is then pumped or bucket-cast into the shell to create a working lining of the trough. Entrained air is removed by immersion vibrators and other mechanical means, thereby creating a low porosity material that conforms to the forms. The forms are removed after the castable hardens.
Forming a working lining from a castable refractory has various challenges, both from an installation and performance perspective. The castable has a limited working time and a viscosity that resists elimination of entrained air. Retained porosity can reduce mechanical performance and make the castable more susceptible to erosion and corrosion. The working lining should have a level surface; however, the geometry of the trough can make this difficult because a trough""s bottom will often be sloped toward the outlets. Unless care is taken, the working lining can slump along the slope.
Use erodes the working lining of the trough. For example, the surface of the molten metal will typically be covered by slag, which can erode the working lining both chemically and mechanically. Slag thermally insulates and protects the metal from contact with air, which can reduce the quality of the finished product. Slag may comprise glasses, fluxes, insulating powders or various impurities. Erosion of the working lining requires that a trough be periodically taken out of service for reconditioning or replacement of the working lining. Downtime is costly, so working linings capable of extended times between reconditioning are very desirable. This requires working linings resistant to slag erosion.
In practice, a working lining can comprise at least two different castable refractories. A first castable is used for contact primarily with molten metal, and a second castable is intended primarily for contact with slag. The first castable often includes a majority of alumina and is cast and leveled into the trough where the iron/slag interface will be. The second castable is formulated to resist slag erosion and is cast on to first castable, preferably before the first castable has set. The second castable is often more costly than the first castable and can, for example, comprise up to 70 wt. % alumina, up to 60 wt. % silicon carbide and smaller amounts of a form of carbon, such as graphite.
Although the use of two castables improves performance, a number of installation difficulties arise from using two castables, including the need for pumping and leveling two castables instead of one, intermixing of the castables if the first castable has not set sufficiently, and formation of a cold joint between the castable layers if the first castable has set prematurely. Intermixing can increase costs without improving performance, and a cold joint is a potential failure point for break-through of the molten metal. Furthermore, the second castable cannot be isolated to specific regions of the trough without erection of a second set of removable forms. In use, xe2x80x9ccold facexe2x80x9d oxidation of the second castable can occur as carbides and carbon present in second castable begin to oxidize on a face of the lining away from the molten metal.
A need persists for a working lining that overcomes these difficulties and for a method of producing the same.
The objective of the present invention is to provide an article and method for creating an erosion-resistant surface in a trough for conveying molten metal. The article includes a trough having a working lining and a pre-cast shape anchored at an erosion zone. The method comprises casting a working lining within the trough and anchoring the precast shape so that the lining and shape define the interior volume of the trough.
The erosion zone can include an impact area where the molten metal enters the trough and any region contacting slag while the molten metal passes through the trough. The precast shape comprises up to 70 wt. % alumina, up to 60 wt. % silicon carbide, and up to 2 wt. % carbon.
The precast shape is described as anchored by mortar, undercuts and mechanical fasteners or, preferably, by combinations thereof. Mechanical fasteners include bolts, screws, T-bars or any other implement to secure the precast shape in the trough.
In another aspect of the invention, a method of manufacturing the trough is described. The trough includes a precast shape anchored at an erosion zone. The method includes installing a form in the trough, casting a working lining into the trough, and anchoring the erosion-resistant precast shape at the erosion zone. The form may be removable and in one embodiment the form includes the precast shape. Conveniently, the form includes undercuts adapted to anchor the precast shape into the trough.